It is often desired to know or determine the location of a device in a network such as a wireless local area network (WLAN). Various techniques for determining location in a WLAN presently exist. For example, a communication may be sent from a first device to a second device that then sends an acknowledgement message back to the first device. The round-trip time (RTT) from the sending of the communication to the receipt of the acknowledgement may be used to determine the distance (which may be an estimated distance) from the second device to the first device, from which location information for the second device may be determined. The RTT, however, includes turnaround time used by the second device to process the communication and send the acknowledgement. If this turnaround time varies, then the determined distance from the second device to the first device will vary.
Referring to FIGS. 1A-1B, a system for determining RTT includes an access point and a mobile device. At a first time, the access point sends Packet 1 to the mobile device that responds by sending an acknowledgement signal ACK 1 to the access point. At a second time, Packet 2 is sent to the mobile device that returns an acknowledgement signal ACK 2 to the access point. An RTT from a time that a packet is sent from a media access control (MAC) of the access point to a time that the acknowledgement reaches the MAC can be determined according to:RX′=RX+MP+CSD  (1)In equation (1), RX is the ideal receive delay, of the acknowledgement, incurred from the antenna of the access point to the MAC of the access point if a cabled signal were presented at the antenna free of multipath and cyclic shift delay (CSD), MP is a timing error attributable to multipath in the receive path, and CSD is the timing error in receiving the acknowledgement by the access point attributable to the cyclic shift delay present at the transmitter of the acknowledgement. RX′ represents the sum of actual delay and timing errors incurred in the scenario of over-the-air reception of the packet in the presence of real multipath and CSD error terms.